import System.Random

isPrime :: Integral a => a -> Bool
isPrime x = and $ fmap ((/=0).(mod x)) [2..(x-1)]

testn = 10

isPrimeT 2 = True
isPrimeT x = and $ fmap ((/=0).(fstPow x (x-1))) $ take testn $ randomRs (2,(x-1)) (mkStdGen 100)

fstPow _ 1 x = x
fstPow m y x 
    | y `mod` 2 == 1 = (x * (fstPow m (y `div` 2) (sqr x))) `mod` m
    | otherwise = fstPow m (y `div` 2) (sqr x)
    where sqr a = (a*a) `mod` m